Look at Liquid Bypass

Such process-side control of tubular exchangers offers advantages.

By Cecil L. Smith, Cecil L. Smith, Inc.

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is the simplest configuration using two
normal control valves.
Click image to enlarge Results of flow simulations for bypass arrangements depend on the ratio of pressure drop across the exchanger and total pressure drop available for fluid flow. This ratio could be very small (the exchanger takes a tiny fraction of the total pressure drop for fluid flow) or essentially 1.0 (all of the pressure drop for fluid flow occurs across the exchanger). This leads to the following two extremes:

Constant total liquid flow (bypass flow plus flow through the exchanger). This occurs when pressure drop across the exchanger is a small portion of total pressure drop available for fluid flow or when a flow controller is installed somewhere in the liquid flow stream.

Constant ΔP across the exchanger. In this case, the exchanger is taking all the pressure drop available for fluid flow.

Operating lines depend on bypass-valve sizing. Figure 2 presents the lines for a valve sized such that when fully open it provides a flow four times that through the exchanger. While not identical, both operating lines for the equal-percentage valve exhibit only modest departures from linearity — certainly not enough to create problems for controller tuning.

When the valve in the bypass line is fully open, liquid flow through the exchanger is at its minimum value. This means the temperature of liquid leaving the exchanger is at its maximum value. Temperature can be computed from flows determined by a simulation. However, it's likely to be very close to the steam supply temperature. This temperature mustn't adversely affect the liquid flowing through the exchanger.

More complex operating lines
Figure 4. More complex operating lines: All lines contain significant nonlinear
characteristics, making tuning harder. Click image to enlarge.
Usually, except when a liquid flow controller is provided, the real situation is somewhere between these two extremes — that is, pressure drop across the exchanger is a significant part but not all of the pressure drop available for fluid flow. To properly analyze this situation, the simulation must encompass the flow system's remaining parts.

Bypass with Two Valves
To achieve a minimum heat-transfer-rate value of zero requires the capability to completely block the flow through the exchanger. This requires using either two normal control valves, one in the bypass line and one in series with the exchanger, or a single three-way valve. Cost is about the same; using two normal control valves usually is preferable.

Several control configurations are possible. Figure 3 illustrates the simplest. One of the control valves is "fail open" while the other is "fail closed." Output from the temperature controller drives both valves. A controller output of 0% passes all flow through the exchanger, providing the maximum liquid outlet temperature. A controller output of 100% bypasses all flow, giving no increase in the liquid temperature. This configuration sometimes is called a "see-saw" arrangement.

Split-range configuration
Figure 5. Split-range configuration:
When controller output is at mid-range,
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